1. Field of the Invention
This invention relates to video data compression.
2. Description of the Prior Art
Some video data compression systems, such as systems defined by the MPEG II standard.sup.1, use a number of different coding techniques to encode successive pictures of a video signal. FNT .sup.1 Motion Picture Experts Group II standard, described in the ISO/IEC Publication DIS 13818/2, "Information Technology--generic coding of motion pictures and associated audio information", March 1995
Typically, the video signal is divided into successive groups of pictures (GOPs). Within each GOP at least one picture is encoded as an "I-picture", or intra-picture, using only information present in that picture itself. This means that I-pictures can later be decoded without requiring information from other pictures, and so provide random entry points into the video sequence. However, the converse of this is that the encoding of I-pictures cannot make use of the similarity between successive pictures, and so the degree of data compression obtained with I-pictures is only moderate.
Further pictures within each GOP may be encoded as "P-pictures" or predicted pictures. P-pictures are encoded with respect to the nearest previous I-picture or P-picture, so that only the differences between a P-picture and the previous P- or I-picture needs to be transmitted. Also, motion compensation is used to encode the differences, so a much higher degree of compression is obtained than with I-pictures.
Finally, some of the pictures within a GOP may be encoded as "B-pictures" or bidirectional pictures. These are encoded with respect to two other pictures, namely the nearest previous I- or P-picture and the nearest following I- or P-picture. B-pictures are not used as references for encoding other pictures, so a still higher degree of compression can be used for B-pictures because any coding errors caused by the high compression will not be propagated to other pictures.
Therefore, in each GOP there are (up to) three classes of picture, I-, P- and B-pictures, which tend to achieve different degrees of compression and so tend to require different shares of the overall available encoded bit stream. Generally, I-pictures require a large share of the available transmission or storage capacity, followed by P-pictures, and followed by B-pictures.
In one previously proposed system, capacity (numbers of bits) in the encoded data stream is allocated between the various pictures in a GOP by a fixed picture to picture ratio. In another scheme, it has been proposed to allocate shares in the available encoded bitstream to individual pictures within a GOP on the basis of the degree of compression obtained for corresponding pictures in a preceding GOP. In this way, the "I:B:P ratio", i.e. the ratio between the numbers of bits allocated to I-, B- and P-pictures in the encoded data stream can be steered in accordance with the image content of the current pictures.
For example, in a sequence of pictures with little or no change between successive pictures, this steering method would result in the available data capacity being concentrated towards the I-pictures. At the opposite extreme, if there is a great deal of image motion between successive pictures, there is likely to be more difference data to be encoded for the B- and P-pictures and so the bit allocation technique would tend to allocate a greater share of the available capacity to B- and P-pictures.
It is a constant aim in the field of video compression to improve the subjective quality of the encoded (and subsequently decoded) images.